Charging Technique for Series Cells

Abstract

An electronic device has a battery system with control circuitry for use during charging and discharging. The battery pack has two battery cells coupled in series. The control circuitry includes battery charging circuitry that applies a charging current to the battery pack during charging. The control circuitry includes bleed resistors and switches that can be selectively activated to bleed charging current away from a selected cell during charging. This allows the control circuitry to balance charges stored on the cells and to balance associated cell voltages. The control circuitry is configured to maintain information on a difference between the charge stored on the first battery cell and the charge stored on the second battery cell. Information on this charge difference value is maintained during charging and discharging and is used in establishing a battery pack charging voltage target.

What is claimed is: 
     
         1 . An electronic device, comprising:
 a battery pack having first and second battery cells coupled electrically in series, wherein the first battery cell stores a first charge and the second battery cell stores a second charge and wherein, when the battery pack is in a quiescent state, the first battery cell is characterized by a first parasitic current and the second battery cell is characterized by a second parasitic current that is different than the first parasitic current; and   control circuitry configured to determine an increase in a difference between the first charge and the second charge due to discharging the battery pack for a discharge period by using information on the first and second parasitic currents.   
     
     
         2 . The electronic device of  claim 1  wherein the control circuitry is configured to:
 determine a decrease in the difference between the first and second charge due to charging the battery pack for a charging period. 
 
     
     
         3 . The electronic device of  claim 2  wherein the control circuitry is configured to control charging of the battery pack using a battery pack charging voltage target. 
     
     
         4 . The electronic device of  claim 3  wherein the control circuitry comprises a first bleed resistor and first switch associated with the first battery cell and a second bleed resistor and second switch associated with the second battery cell. 
     
     
         5 . The electronic device of  claim 4  wherein the control circuitry is configured to close a selected one of the first and second switches to switch into use a selected one of the first and second bleed resistors during charging when a threshold state of charge on a selected one of the first and second battery cells is exceeded. 
     
     
         6 . The electronic device of  claim 5  wherein the control circuitry is configured to determine the battery pack charging voltage target to use based at least partly on information on when the threshold state of charge is exceeded. 
     
     
         7 . Battery circuitry, comprising:
 first and second battery cells coupled in series between a first terminal and a second terminal; and   control circuitry configured to measure a voltage across the first and second terminals and configured to track changes in a difference in charge between the first and second battery cells without measuring individual voltages on the first and second battery cells.   
     
     
         8 . The battery circuitry of  claim 7  wherein the first battery cell stores a first charge and the second battery cell stores a second charge, wherein when the battery pack is in a quiescent state the first battery cell is characterized by a first parasitic current and the second battery cell is characterized by a second parasitic current that is different than the first parasitic current, and wherein the control circuitry is configured to maintain information on a difference between the first and second charges during charging and discharging of the first and second battery cells. 
     
     
         9 . The battery circuitry of  claim 8  wherein the control circuitry is configured to determine an amount to increase the difference between the first and second charges during discharging of the first and second battery cells. 
     
     
         10 . The battery circuitry of  claim 9  wherein the control circuitry is configured to determine the amount to increase the difference between the first and second charges based on a duration associated with the discharging. 
     
     
         11 . The battery circuitry of  claim 10  wherein the control circuitry comprises switching circuitry and bleed resistors configured to balance the first and second battery cells during charging. 
     
     
         12 . The battery circuitry of  claim 11  wherein the control circuitry is configured to maintain the information on the difference between the first and second charges at least partly by determining when bleed resistors are switched into use during charging and at least partly by measuring charging duration. 
     
     
         13 . The battery circuitry of  claim 12  wherein the first and second battery cells form a battery pack and wherein the control circuitry is configured to use the information maintained on the difference between the first and second charges in determining a battery pack charging voltage target to use in charging the battery pack. 
     
     
         14 . The battery circuitry of  claim 12  wherein the control circuitry is configured to determine the battery pack charging voltage target using the information maintained on the difference between the first and second charges, charging duration information, and open-circuit-voltage-versus-charge information for the cells. 
     
     
         15 . A battery system for an electronic device, comprising:
 a battery pack formed from first and second battery cells coupled in series between first and second battery pack terminals;   a first bleed resistor and first switch coupled across the first battery cell;   a second bleed resistor and second switch coupled across the second battery cell;   a voltage sensor coupled between the first and second battery pack terminals;   control circuitry that includes charging circuitry configured to supply a charging current that flows through the battery pack between the first and second terminals to charge the battery pack, wherein the control circuitry is configured to:
 control the first and second switches during charging of the battery pack to short a selected one of the first and second bleed resistors across a selected one of the first and second battery cells; and 
 control the charging circuitry to charge the battery pack to a target battery pack charging voltage target determined at least partly based on information on a difference between a first charge on the first battery cell and a second charge on the second battery cell. 
   
     
     
         16 . The battery system of  claim 15  wherein the control circuitry is configured to use open-circuit-voltage-versus-charge information for the first and second battery cells and the information on the difference between the first charge on the first battery cell and the second charge on the second battery cell in determining the battery pack charging voltage target. 
     
     
         17 . The battery system of  claim 15  wherein the control circuitry is configured to maintain the information on the difference between the first charge on the first battery cell and the second charge on the second battery cell during charging and discharging of the battery pack. 
     
     
         18 . The battery system of  claim 17  wherein the control circuitry is configured to compute an updated value of the difference between the first charge on the first battery cell and the second charge on the second battery cell following a discharge period using information on discharge period duration. 
     
     
         19 . The battery system of  claim 18  wherein the control circuitry is configured to compute the updated value of the difference between the first charge on the first battery cell and the second charge on the second battery cell following the discharge period using information on a first parasitic current drawn from the first battery cell through the control circuitry when the control circuitry is in a quiescent state and a second parasitic current drawn from the second battery cell through the control circuitry when the control circuitry is in the quiescent state. 
     
     
         20 . The battery system of  claim 17  wherein the control circuitry is configured to compute an updated value of the difference between the first charge on the first battery cell and the second charge on the second battery cell following a charge period using information on bleed resistor use during charging.